Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative effect, period of adoption, adjustment	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Retained Earnings	Retained Earnings Cumulative effect, period of adoption, adjustment	Total Tetra Tech Equity	Total Tetra Tech Equity Cumulative effect, period of adoption, adjustment	Non-Controlling Interests
Beginning balance (in shares) at Sep. 30, 2018			55,349,000
Beginning balance at Sep. 30, 2018	$ 967,100	$ (2,766)	$ 553	$ 148,803	$ (127,350)	$ 944,965	$ (2,766)	$ 966,971	$ (2,766)	$ 129
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	147,227					147,141		147,141		86
Other comprehensive income (loss)	(19,486)				(19,728)			(19,728)		242
Distributions paid to noncontrolling interests	(276)									(276)
Cash dividends per common share	(21,489)					(21,489)		(21,489)
Stock-based compensation	12,717			12,717				12,717
Restricted & performance shares released (in shares)			180,000
Restricted & performance shares released	(6,840)		$ 2	(6,842)				(6,840)
Stock options exercised (in shares)			293,000
Stock options exercised	7,759		$ 3	7,756				7,759
Shares issued for Employee Stock Purchase Plan (in shares)			148,000
Shares issued for Employee Stock Purchase Plan	6,847		$ 2	6,845				6,847
Stock repurchases (in shares)			(1,256,000)
Stock repurchases	(75,000)		$ (13)	(74,987)				(75,000)
Ending balance (in shares) at Jun. 30, 2019			54,714,000
Ending balance at Jun. 30, 2019	1,015,793		$ 547	94,292	(147,078)	1,067,851		1,015,612		181
Beginning balance (in shares) at Mar. 31, 2019			54,947,000
Beginning balance at Mar. 31, 2019	990,606		$ 549	111,277	(148,217)	1,026,837		990,446		160
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	49,251					49,233		49,233		18
Other comprehensive income (loss)	1,142				1,139			1,139		3
Cash dividends per common share	(8,219)					(8,219)		(8,219)
Stock-based compensation	4,122			4,122				4,122
Restricted & performance shares released (in shares)			1,000
Restricted & performance shares released	(33)		$ 1	(34)				(33)
Stock options exercised (in shares)			142,000
Stock options exercised	3,924		$ 1	3,923				3,924
Stock repurchases (in shares)			(376,000)
Stock repurchases	(25,000)		$ (4)	(24,996)				(25,000)
Ending balance (in shares) at Jun. 30, 2019			54,714,000
Ending balance at Jun. 30, 2019	1,015,793		$ 547	94,292	(147,078)	1,067,851		1,015,612		181
Beginning balance (in shares) at Sep. 29, 2019			54,565,000
Beginning balance at Sep. 29, 2019	989,464		$ 546	78,132	(160,584)	1,071,192		989,286		178
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	129,234					129,205		129,205		29
Other comprehensive income (loss)	(17,154)				(17,148)			(17,148)		(6)
Distributions paid to noncontrolling interests	(69)									(69)
Cash dividends per common share	(25,590)					(25,590)		(25,590)
Stock-based compensation	13,494			13,494				13,494
Restricted & performance shares released (in shares)			210,000
Restricted & performance shares released	(11,143)		$ 2	(11,145)				(11,143)
Stock options exercised (in shares)			283,000
Stock options exercised	8,263		$ 3	8,260				8,263
Shares issued for Employee Stock Purchase Plan (in shares)			168,000
Shares issued for Employee Stock Purchase Plan	8,715		$ 1	8,714				8,715
Stock repurchases (in shares)			(1,338,000)
Stock repurchases	(102,188)		$ (13)	(97,455)		(4,720)		(102,188)
Ending balance (in shares) at Jun. 28, 2020			53,888,000
Ending balance at Jun. 28, 2020	993,026		$ 539	0	(177,732)	1,170,087		992,894		132
Beginning balance (in shares) at Mar. 29, 2020			54,142,000
Beginning balance at Mar. 29, 2020	950,238		$ 541	10,473	(199,391)	1,138,485		950,108		130
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	45,503					45,497		45,497		6
Other comprehensive income (loss)	21,655				21,659			21,659		(4)
Cash dividends per common share	(9,175)					(9,175)		(9,175)
Stock-based compensation	4,057			4,057				4,057
Restricted & performance shares released (in shares)			(5,000)
Restricted & performance shares released	(45)		$ 0	(45)				(45)
Stock options exercised (in shares)			14,000
Stock options exercised	336		$ 0	336				336
Stock repurchases (in shares)			(263,000)
Stock repurchases	(19,543)		$ (2)	(14,821)		(4,720)		(19,543)
Ending balance (in shares) at Jun. 28, 2020			53,888,000
Ending balance at Jun. 28, 2020	$ 993,026		$ 539	$ 0	$ (177,732)	$ 1,170,087		$ 992,894		$ 132